[lld] [ELF] Add --print-gc-sections=<file> (PR #159706)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> LGTM. Always good to have the flexibility to write to a file.
> 
> Above printGCSections, there is printIcfSections. Although not for this patch, would it be useful to apply the same logic to that too?

Thanks  for the review! Yes:) Will create a separate patch for --print-icf-sections= 

https://github.com/llvm/llvm-project/pull/159706